The presence of common mode signals in instrumentation systems that are primarily interested in monitoring differential mode signals is a common phenomenon. A paradigm of such a system is a heart monitor and defibrillator. The monitor is primarily interested in detecting the differential mode voltage signals of a patient's heart and displaying it as a familiar electrocardiogram (ECG) reading. Nevertheless, interfering signals can be introduced into such systems that can masquerade as differential signals of interest. One source of such interfering signals is called "common-mode" signals which can manifest system outputs as if arising from differential sources, such as the patient's heart. The introduction of these common mode signals into such a system is a nuisance at best to the optimal functioning of the system, and at worst, potentially harmful to the patient under certain circumstances.
Referring to FIG. 1, a simplified illustration of how common mode signals are introduced into a typical instrumentation system, such as system 10, is depicted. System 10 consists of electrodes 16a and 16b electrically connecting patient 12 to instrumentation package 14. Instrumentation 14 is used to either detect certain patient conditions or to effect changes in a patient's state or both. In the case of the heart monitor and defibrillator, the monitor detects a state of fibrillation in a patient's heart and the defibrillator administers a electric shock to return the patient's heart to a non-fibrillating state.
The introduction of a common mode signal into this system can lead to either false positive indications of patient fibrillation when the patient's heart is actually functioning properly or false negative indications of a normal patient when the patient is actually experiencing fibrillation. Either situation can have very serious consequences--inducing a shock from the defibrillator when none is needed in the first instance or inducing inaction in the defibrillator when a life-saving shock is needed in the second instance.
A complete common mode circuit is depicted in FIG. 1. Typically, an outside common mode voltage source 18 drives a common mode current 20 from earth ground 22 through patient 12 and instrumentation 14. Stray capacitance between the earth and patient and the earth and instrumentation (represented by capacitors 24 and 26) completes the common mode circuit. Changes in either the common mode voltage source or changes in stray capacitance 24 and 26 may induce a flow of common mode current 20.
A more detailed situation involving common mode signals is depicted in FIG. 2. In this case, the patient/instrumentation system 10 is subjected to a number of different sources of common mode signals. Common mode voltages may arise from a number of sources--for example, from static electricity arising from various interactions such as between patient 12 and earth 22, instrumentation 14 and earth 22, an individual 28 aiding the patient and earth, as well as from varying electric fields such as overhead lighting 36 and other electrical sources. Each of these interactions have their own common mode voltage and associated capacitance.
In the case of heart monitoring for a defibrillator, the common mode signals generated from these various sources may have a significant impact on the qualitative and quantitative analysis of a true electrical signal generated by the heart of a patient. The main reason is that the electrical signal of the heart is comparatively small. By contrast, the common mode signals generated from the sources mentioned above may vary in intensity and may be orders of magnitude larger.
Sometimes a common mode signal may be so strong as to dominate the "overall" signal (i.e. the output resulting from the common mode signal superimposed over the output signal resulting from the differential signal from the patient's heart) received at the instrumentation. If such an overpowering common mode signal were to vary at the rate of a few Hertz (as many common mode signals do), then the instrumentation may interpret the received signal as a heart in a state of fibrillation. This false positive reading might induce the instrumentation to deliver an electric shock to a patient whose heart is actually operating properly--a confusing and potentially harmful situation.
Likewise, a large common mode signal may be generated with a pattern similar to a correctly beating heart. In such a case, the instrumentation may interpret the received signal as a heart in a normal beating state. This false negative reading would not enable the defibrillator to deliver a life-saving shock to a patient whose heart has stopped beating properly--which may result in a failure to resuscitate the victim.
Actual test situations and their results are shown in FIGS. 3A and 3B, 4A and 4B, and 5A and 5B. In these tests, a common mode current was injected into a heart monitoring system that was concurrently monitoring the heart rhythms of a pig used as a test subject. Common mode current, induced into the monitoring system, is graphed as a function of time in FIGS. 3A, 4A, and 5A. The signal received (i.e. a superposition of the common mode signal and heart signal) by the instrumentation in each case is graphed in FIGS. 3B, 4B, and 5B. As will be discussed in greater detail in connection to FIG. 6 below, the common mode signal being superimposed is a combination of common mode signal that is "converted" to a differential mode signal (due to impedance mismatching) and common mode signal that is passed through an actual amplifier.
In FIG. 3A, a negligible common mode signal was injected into the system while the pig's heart was beating normally. The received signal in FIG. 3B therefore registers a normal heart beat and the monitoring system therefore correctly interprets this signal to be a normally beating heart--one not needing to be shocked.
FIGS. 4A and 4B depict a "false positive" reading in the heart monitoring instrumentation. In FIG. 4A, a much stronger common mode current in injected at a varying rate of fluctuation. In this instance, this common mode signal is introduced at a time when the subject pig's heart is beating normally. However, since the common mode signal is much stronger than the subject heart signal, the received signal detected in FIG. 4B is dominated by the common mode signal. A likely diagnosis from a diagnostic system in such a case would be a state of fibrillation in the subject's heart and to recommend to the operator that a shock be delivered.
FIG. 5A and 5B depict another "false positive" reading. In FIG. 5A, the common mode signal arose as a result of an attempt at cardiopulmonary resuscitation (CPR) at a time when the subject pig's heart was beating normally. The CPR induced a common mode signal that has a look similar to a normal beating heart. However, when that signal is superimposed on the signal of an actual beating heart, the signal received in FIG. 5B may resemble a heart that is in a state of fibrillation when in fact the subject heart is beating normally.
It is thus highly desirable to alleviate the misdiagnosis of superimposed common mode signals. It is therefore an object of the present invention to design an instrumentation system, primarily interested in detecting differential mode signals, that detects any superimposed common mode signals.